I too also used a metal spudger on the left side opposite the volume rocker, that seems like a good spot to start.
I found that on my HTC M8 the power button needs to be a shade to the left. You can figure it out on your model by looking for the button by using a Sim Card eject tool and pressing on the power button which on my model was covered by black tape. You’ll feel the button click/press once you find it - you can then determine if the button is a shade to the left or right when you line it up with the housing with no power button. Place the phone temporarily into the shell and you’ll be able to determine the button placement, this is how I did it.